This invention relates to a detector of passing ferromagnetic articles such as gear teeth, the detector being made up of a pair of Hall devices differentially connected and assembled at a pole end of a magnet, and more particularly pertains to such a detector having interposed, between the magnet and Hall devices, a ferromagnetic pole piece that is thinner at the center than at the periphery for providing a uniform magnetic field.
It is known to use two differentially connected Hall elements mounted at a pole end of a magnet as a ferromagnetic article proximity detector. In the patent U.S. Pat. No. 4,518,918 to G. Avery, issued May 21, 1985 and assigned to the same assignee as is the present invention, such a detector is employed as a gear tooth sensor wherein two separate Hall devices are mounted directly to the magnet pole end and spaced apart a distance equally about half the pitch of the gear teeth. In a related sensor assembly of a magnet and a plurality of magneto resistors, a ferromagnetic plate is interposed between the resistors and the magnet so that the resistors are in a more uniform field.
In a patent by J. K. Higgs and B. L. Gibson, U.S. Pat. No. 4,859,941 issued Aug. 22, 1989 and assigned to the same assignee as is the present invention, there is described an assembly of a magnet, a pole piece and dual Hall IC. The pole piece has extensions that turn downward alongside the magnet to improve the uniformity of the field across the magnet pole and to increase the working range of gap dimensions between the assembly and a rotating ferromagnetic gear, the teeth of which are to be detected by the assembly.
It is an object of this invention to provide in such a ferromagnetic-article proximity sensor an even more uniform pole end field and further to provide an improved tolerance for large gaps between the dual Hall sensor and a passing ferromagnetic article to be detected.